近红外光响应液晶弹性体

刺激-响应液晶弹性体是一类新兴的智能聚合物材料,其在外界刺激(热、光、电、磁场等)下会产生大尺寸的可逆形变,因此具有广阔的应用前景。由于单轴取向的液晶基元的微观顺序或分子结构的变化,整个液晶弹性体材料在液晶相向各向同性相转变过程中可以发生非常大的可逆宏观形变。其中,由于近红外光的强穿透力和对生物组织的低毒性,近红外光响应液晶弹性体受到了科学家们的广泛关注。近红外光响应液晶弹性的变形机制主要分为两大类。一种是通过掺杂无机或有机上转换材料将近红外光转化为低波长的光,激发偶氮苯发生顺反异构化。另一种近红外光响应液晶弹性体利用导热填料的光热效应将光转化为热,从而进一步诱导液晶相向各向同性相转变,从而使液晶弹性体材料发生形变。这些优点使近红外光响应液晶弹性体具有潜在的应用价值,如驱动器、人造器官、智能表面和微型机器人等。本文综述了近红外激光响应材料的研究进展,详细介绍了近红外光响应材料的主要变形机理及其应用,并对近红外光响应液晶弹性体和软驱动器的发展前景进行了展望。     

液晶弹性体刺激形变研究

开发可以通过外部刺激产生机械形变的人工致动材料是一个近年来的研究热点。其中,液晶弹性体因结合了聚合物网络的橡胶弹性和液晶的有序性而具有独特的性质,在热、光、电等的外界刺激下可以产生可逆的形状记忆效应。本文综述了液晶弹性体响应多种外界刺激产生各种形变的行为,主要介绍了有关热致形变液晶弹性体、电致形变液晶弹性体、化学刺激导致形变的液晶弹性体及光致形变液晶弹性体的研究进展,阐述了各类液晶弹性体产生形变的机理包括热致、电致和光致相转变,讨论了影响其响应性能的主要因素,并展望了这一领域的发展前景。     

室温光致形变主链型交联液晶高分子纤维执行器

将分别含有偶氮苯和苯甲酸苯酯的2种液晶单体与伯胺进行迈克尔加成反应,得到分子量可控的主链型液晶低聚物.利用先熔融提拉、后自由基聚合的方法将低聚物制备成交联液晶高分子纤维.液晶单体与伯胺之间的扩链反应有效地降低了高分子网络的交联密度,进而降低纤维的玻璃化转变温度.同时,这种先拉伸、再聚合的方式将纤维的取向与交联过程分开,保证了纤维中的液晶基元沿拉伸方向呈规整排列.因此,取向后的交联液晶高分子纤维可在室温下实现可逆光致弯曲形变,其最大弯曲角度接近60°,且弯曲方向可以通过改变光照方向进行调控.这种形变幅度大、方向可控的主链型交联液晶高分子纤维有望推动光响应柔性执行器等领域的发展.     

腰接型侧链液晶弹性体研究进展

液晶弹性体是交联型液晶大分子,兼具液晶取向有序性和交联聚合物熵弹性等特性,在传感器、触发器、微流体装置和仿生器件等方面具有很好的应用前景.制备液晶弹性体的微结构,探索其独特的刺激响应性,是目前液晶弹性体研究的重要方向.侧链液晶弹性体的液晶相态类型取决于其液晶基元和主链的连接方式.腰接型侧链液晶弹性体倾向于形成向列型液晶相,具有较快的响应速度和形变程度,是一类独特的液晶弹性体.本文重点介绍腰接型液晶弹性体微结构(如微米柱、微米线等)的制备;利用金纳米粒子的光热转换效应,实现液晶弹性体光响应性的新途径;以及腰接型侧链液晶弹性体仿生微结构的功能性等.同时还对该领域的发展前景进行了展望.     

光致变形型高分子材料

光致变形型高分子材料以光为激发源,在没有机械接触的情况下,能够快速改变尺寸和形状。本文介绍了光致变形的高分子凝胶、无定形高分子、液晶弹性体和光致形状记忆高分子材料,并对各种材料的光致变形机理进行解释。无定形高分子的光致变形较小,目前研究重点是具有各向异性的液晶弹性体。文中着重介绍了具有偶氮苯介晶基元的液晶弹性体的光致变形研究,在光照下这类材料只要有1%的偶氮苯介晶基元发生顺反异构,就会发生光致变形。     

光致形变液晶高分子

交联液晶高分子兼具液晶的各向异性和高分子网络的弹性,并且具有优异的分子协同作用.在交联液晶高分子中引入光响应基团,例如偶氮苯后,即可赋予其光致形变性能,利用分子协同作用可以将光化学反应引起的分子结构变化放大为宏观形变,从而将光能直接转化成机械能.通过合理的分子结构和取向设计可以使液晶高分子产生诸如伸缩、弯曲、扭曲、振动等多种形式的光致形变,并用于各类光控柔性执行器件的构筑,在人工肌肉、微型机器人、微量液体操控等领域呈现出独特的优势和广阔的应用前景.本文总结和评述了光致形变液晶高分子的研究,包括材料结构对光致形变性能的影响、新型可加工光致形变材料的研究、利用可见光和近红外光触发形变的策略,以及光致形变液晶高分子微执行器在微量液体操控中的应用,最后展望了该领域的发展方向.     

双酚-F不饱和液晶化合物的合成与表征

以4,4'-二羟基二苯基甲烷(双酚-F),3-溴1-丙烯(或6-溴1-己烯)为原料合成了p-BAMBP,p-MBPAB和p-MBPHB三种新型含有不同长度不饱和末端基的液晶化合物,并用POM﹑FTIR、1H-NMR、DSC和XRD进行了表征.结果表明它们均为近晶型液晶,且熔点随柔性末端基增长而降低,相转变温度范围变宽.此类化合物有望作为新型液晶化合物或设计新型液晶功能高分子材料使用.     

可多次塑型、易修复及耐低温的三维动态高分子结构

三维动态高分子结构可在外界刺激下发生可逆的形状改变,但仅采用通常的平面高分子材料,其形成的三维动态结构形成后难以更改,损坏后不易修复,低温下无法使用.最近,将碳纳米管与液晶型类玻璃高分子结合,制备了新型的纳米复合材料薄膜,通过光照对平面材料进行加工,不仅可实现三维动态结构的灵活设计和简易制备,而且结构可多次改变和修复,在低至零下130℃的条件下仍可以使用.     

环氧液晶弹性体材料的制备与力学性能研究

传统的液晶弹性体材料多采用丙烯酸酯类或聚硅氧烷类分子,通过自由基聚合制备.然而由于自由基聚合易被氧气阻聚,固化过程收缩率高且内应力大,传统液晶弹性体材料的力学性能并不是很优异.为解决这一问题,本文合成了带有环氧基团的液晶单体和交联剂,使用碘鎓盐作为引发剂,通过光引发阳离子反应,用原位聚合交联法制备了环氧液晶弹性体材料.光引发阳离子聚合和传统自由基聚合相比,不受氧气影响且弹性体固化时体积收缩率小.对制备出的液晶弹性体材料的晶相和力学等性能进行探究和表征,发现其比传统的丙烯酸酯类或者聚硅氧烷类液晶弹性体材料具有更优异的力学性能,在清亮点以上,其弹性模量达到了0.92 MPa.     

预取向液晶单体光聚合反应

通过表面摩擦、施加外电场(或磁场)和应力剪切等手段使液晶单体取向后进行光聚合反应是制备高度取向、高度均匀和高度透明高分子液晶膜的重要方法。它在高分子非线性光学材料、导电商分子材料和光纤涂层等领域中具有广泛的应用前景。     

A Novel Side-Chain Liquid Crystal Elastomer Exhibiting Anomalous Reversible Shape Change

Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli-controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)-isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side-chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC-disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side-group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit.     

A Novel Side‐Chain Liquid Crystal Elastomer Exhibiting Anomalous Reversible Shape Change

Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli‐controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)‐isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side‐chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC‐disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side‐group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit.     

Liquid crystal elastomer actuators: Synthesis,alignment, and applications

Liquid crystal elastomers (LCEs) are a unique class of materials which combine rubber elasticity with the orientational order of liquid crystals. This combination can lead to materials with unique properties such as thermal actuation, anisotropic swelling, and soft elasticity. As such, LCEs are a promising class of materials for applications requiring stimulus response. These unique features and the recent developments of the LCE chemistry and processing will be discussed in this review. First, we emphasize several different synthetic pathways in conjunction with the alignment techniques utilized to obtain monodomain LCEs. We then identify the synthesis and alignment techniques used to synthesis LCE‐based composites. Finally, we discuss how these materials are used as actuators and sensors. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 395–411     

One-Step Manufacturing of Supramolecular Liquid-Crystal Elastomers by Stress-Induced Alignment and Hydrogen Bond Exchange

Liquid-crystal elastomers (LCEs) capable of performing large and reversible deformation in response to an external stimulus are an important class of soft actuators. However, their manufacturing process typically involves a multistep approach that requires harsh conditions. For the very first time, LCEs with customized geometries that can be manufactured by a rapid one-step approach at room temperature are developed. The LCEs are hydrogen bond (H-bond) crosslinked main chain polymers comprising flexible short side chains. Applying a stretching/shear force to the LCE can simultaneously induce mesogen alignment and H-bond exchange, allowing for the formation of well-aligned LCE networks stabilized by H-bonds. Based on this working principle, soft actuators in fibers and 2D/3D objects can be manufactured by mechanical stretching or melt extrusion within a short time (e.g. <1 min). These actuators can perform reversible macroscopic motions with large, controlled deformations up to 38 %. The dynamic nature of H-bonds also provides the actuators with reprocessability and reprogrammability. Thus, this work opens the way for the one-step and custom manufacturing of soft actuators.     

Photo-responsive liquid crystalline elastomer with reduced chemically modified graphene oxide

Photo-responsive liquid crystalline elastomer (LCE) with reduced chemically modified graphene oxide (RMGO) was fabricated and studied. Mesogenic groups modified graphene oxide (MGO) was prepared, reduced and characterised, then the obtained RMGO was mixed with an acrylate monomer containing a side-on mesogen, a crosslinker and a photoinitiator. After being oriented with magnetic field, well-defined LCE micropillar as photo-responsive actuators were fabricated from the mixture by the method combining soft lithography and photo-polymerisation/photo-crosslinking. The LCE micropillars showed reversible thermo-mechanical deformation during the nematic-to-isotropic transition temperature. Upon irradiating with red light (650 nm), photo-mechanical responses of the RMGO-containing LCE was demonstrated. This micron-sized LCE actuators can be used for remote photo-responsive devices.     

Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: a step towards coarse graining

Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling.     

Liquid‐Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.     

Synthesis and curing of novel LC twin epoxy monomers for liquid crystal thermosets

This article describes the synthesis and characterization of new liquid crystalline thermosets having a twin structure. Nematic epoxy-terminated monomers based on a phenyl benzoate twin mesogen connected by an alkylene spacer were synthesized for these studies. In addition, an epoxy-terminated monomer based on a 1,4-bis(benzoyloxy)phenylene mesogen was synthesized to determine the effect of the position of the mesogen on the final network structure. The diepoxy monomer made with phenyl benzoate twin mesogens connected with an alkylene spacer formed a smectic-like network when cured with diamines. This smectic organization appeared even though the diepoxy monomer itself showed only a nematic mesophase over a narrow temperature range. The presence of crosslinks at both ends of the mesogens helped to retain a uniform spacing between crosslinking sites during the curing reaction, and aided formation of the smectic layer arrangement. The epoxy monomer possessing a 1,4-bis(benzoyloxy)phenylene mesogen and two epoxidized alkylene end groups on both sides of the mesogen formed a stable nematic mesophase. However, in contrast to the twin epoxies, the latter epoxy when reacted with diamines tended to produce a nematic-like network which was retained as the crosslinking reaction proceeded. © 1996 John Wiley & Sons, Inc.     

Synthesis and curing of liquid crystalline epoxy resin based on naphthalene mesogen

Aromatic liquid crystalline epoxy resin (LCE) based on naphthalene mesogen was synthesized and cured with aromatic diamines to prepare heat‐resistant LCE networks. Diaminodiphenylester (DDE) and diaminodiphenylsulfone (DDS) were used as curing agents. The curing reaction and liquid crystalline phase of LCE were monitored, and mechanical and thermal properties of cured LCE network were also investigated. Curing and postcuring peaks were observed in dynamic DSC thermogram. LCE network cured with DDE displayed liquid crystalline phase in the curing temperature range between 183 and 260°C, while that cured with DDS formed one between 182 and 230°C. Glass transition temperature of cured LCE network was above 240°C, and crosslinked network was thermally stable up to 330°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 419–425, 1999